1. Field of the Invention
This invention relates to a system for fabricating passive resistors in integrated circuits that display minimal change in resistance over a wide range of operating temperatures.
2. Brief Description of the Prior Art
Integrated circuits are generally fabricated with polysilicon resistors that are formed on the semiconductor substrate. Such resistors generally have a resistor body, generally formed of doped polysilicon and generally having metallic leads coupled to opposing ends of the resistor body, generally through contacts in field oxide. The contacts are connected to metal interconnect. The resistor body can be formed concurrently with polysilicon transistor gate electrodes, such resistor body generally doped and generally resting on the field oxide.
Integrated circuits that require passive resistors often have tight tolerances on the resistance value of these resistors. However, these prior art semiconductor resistors are subject to variations in resistance value. Sources of variation in the resistance value of these resistors include process fluctuations that result in physical changes to the resistor properties such as physical dimension or resistivity and changes in temperature. The sources of change in temperature can be either external to the resistor itself or internal due to the self-heating effects associated with power dissipation. As the resistor temperature changes, the value of resistance of the resistor also changes.
The general prior art method utilized for minimizing the resistance alteration effects due to the temperature coefficient of resistance (TCR) of a semiconducting resistor (a resistor formed of semiconductor material) is to increase the doping concentration in the resistor body to a sufficiently high level such that the TCR of the resistor body is at a minimum. Then the resistors are built with dimensions that make the resistor head resistance a small percentage of the resistor body resistance. As a result, the resistor head TCR has little effect on the overall resistor temperature characteristics.
To reach sufficiently low TCR conditions in the resistor body alone, the impurity or doping concentration must be very high, about 3.times.10.sup.20 atoms/cm.sup.3 for polysilicon resistors. Extra processing steps are often required to reach this level of impurity concentration. These steps add cost to the production of the circuit and limit the range of resistance values obtainable in that the sheet resistance (R.sub.s) is low (about 70 to about 100 ohms/square). The simple expression for resistance is R=R.sub.s (L/W), where L is length and W is width of the resistor body. This equation shows that to achieve the desired value of R when R.sub.s is low, W must be minimized (which increases the head component of resistance and increases process variability) and/or L must be increased, the latter increasing capacitance and area consumed on the chip. The increase in length is also detrimental at high frequency where these resistors are sometimes used.